This invention relates generally to ultrasound diagnostic systems that use ultrasonic transducers to provide diagnostic information concerning the interior of the body through ultrasound imaging, and more particularly, to micro-machined ultrasonic transducers used in such systems.
Ultrasonic diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements. For example, cardiologists, radiologists, and obstetricians use ultrasonic diagnostic imaging systems to examine the heart, various abdominal organs, or a developing fetus, respectively. In general, imaging information is obtained by these systems by placing an ultrasonic probe against the skin of a patient, and actuating an ultrasonic transducer located within the probe to transmit ultrasonic energy through the skin and into the body of the patient. In response to the transmission of ultrasonic energy into the body, ultrasonic echoes emanate from the interior structure of the body. The returning acoustic echoes are converted into electrical signals by the transducer in the probe, which are transferred to the diagnostic system by a cable coupling the diagnostic system to the probe.
Acoustic transducers commonly used in ultrasonic diagnostic probes are comprised of an array of individual piezoelectric elements formed from a piezoelectric material by the application of a number of meticulous manufacturing steps. In one common method, a piezoelectric transducer array is formed by bonding a single block of piezoelectric material to a backing member that provides acoustic attenuation. The single block is then laterally subdivided by cutting or dicing the material to form the rectangular elements of the array. Electrical contact pads are formed on the individual elements using various metallization processes to permit electrical conductors to be coupled to the individual elements of the array. The electrical conductors are then coupled to the contact pads by a variety of electrical joining methods, including soldering, spot-welding, or by adhesively bonding the conductor to the contact pad.
Although the foregoing method is generally adequate to form acoustic transducer arrays having up to a few hundred elements, larger arrays of transducer elements having smaller element sizes are not easily formed using this method. Consequently, various techniques used in the fabrication of silicon microelectronic devices have been adapted to form ultrasonic transducer elements, since these techniques generally permit the repetitive fabrication of small structures in intricate detail.
An example of a device that may be formed using semiconductor fabrication methods is the micro-machined ultrasonic transducer (MUT). The MUT has several significant advantages over conventional piezoelectric ultrasonic transducers. For example, the structure of the MUT generally offers more flexibility in terms of optimization parameters than is typically available in conventional piezoelectric devices. Further, the MUT may be conveniently formed on a semiconductor substrate using various semiconductor fabrication methods, which advantageously permits the formation of relatively large numbers of transducers, which may then be integrated into large transducer arrays. Additionally, interconnections between the MUTs in the array and electronic devices external to the array may also be conveniently formed during the fabrication process. MUTs may be operated capacitively, and are referred to as cMUTs, as shown in U.S. Pat. No. 5,894,452. Alternatively, piezoelectric materials may be used to fabricate the MUT, which are commonly referred to as pMUTs, as shown in U.S. Pat. No. 6,049,158. Accordingly, the MUT has increasingly become an attractive alternative to conventional piezoelectric ultrasonic transducers in ultrasound systems.
FIG. 1 is a partial cross sectional view of a MUT 1 according to the prior art. The MUT 1 may have a platform that is rectangular, circular, or may be of other regular shapes. The MUT 1 generally includes an upper surface 2 that is spaced apart from a lower surface 3 that abuts a silicon substrate 5. Alternatively, a dielectric layer 4 may be formed on the substrate 5 that underlies the MUT 1. When a time-varying excitation voltage (not shown) is applied to the MUT 1, a vibrational deflection in the upper surface 2 is developed that stems from the electro-mechanical properties of the MUT 1. Accordingly, acoustic waves 6 are created that radiate outwardly from the upper surface 2 in response to the applied time-varying voltage. The electro-mechanical properties of the MUT 1 similarly allow the MUT 1 to be responsive to deflections resulting from acoustic waves 7 that impinge on the upper surface 2.
One disadvantage in the foregoing prior art device is that a portion of the ultrasonic energy developed by the MUT 1 may be projected backwardly into the underlying substrate 5, rather that being radiated outwardly in the acoustic wave 6, which results in a partial loss of radiated energy from the MUT 1. Moreover, when ultrasonic energy is coupled into the underlying substrate 5, various undesirable effects are produced, which are briefly described below.
With reference now to FIG. 2, a partial cross sectional view of a MUT array 10 according to the prior art is shown. The array 10 includes a plurality of MUT transducers 1 formed on a silicon substrate 5. Each transducer 1 is coupled to a time-varying voltage source through a plurality of electrical interconnections formed in the substrate 5. For clarity of illustration, the voltage source and the electrical interconnections are not shown. An acoustic wave 21 may be conducted into the substrate 5 through a back surface 3. The wave 21 propagates within the substrate 5 and is internally reflected at a lower surface 18 of the substrate 5 to form a reflected wave 23 that is directed towards an upper surface 19 of the substrate 5. Consequently, a plurality of reflected waves 23 propagate within the substrate 5 between the upper surface 19 and the lower surface 18. A portion of the energy present in each reflected wave 23 may also leave the substrate 5 through the surface 18, to form a plurality of leakage waves 25. An internal reflection 27 from an end 24 of the array 10 may lead to still further reflected waves 27 and leakage waves 26.
The propagation of acoustic waves 23 and 27 in the substrate 5, as described above, permits ultrasonic energy to be cross-coupled between the plurality of MUT transducers 1 on the substrate 5 and produce undesirable xe2x80x9ccross-talkxe2x80x9d signals between the plurality of MUTs 1, as well as other undesirable interference effects. Still further, the internal reflection of waves in the substrate 5 may adversely affect the acceptance angle, or directivity of the array 10.
Various prior art devices have included elements that impede the propagation of waves in the substrate. For example, one prior art device employs a plurality of trenches between the MUTs 1 that extend downwardly into the substrate 5 to interrupt wave propagation within the substrate 5. Another prior art device employs a similar downwardly projecting trench, and fills the trench with an acoustic absorbing material in order to at least partially absorb the energy in the reflected waves 23. Other prior art devices minimize lateral wave propagation by controlling still other geometrical details of the array. Although these prior art devices generally reduce the undesired lateral wave propagation in the substrate, they generally limit the design flexibility inherent in the MUT by reducing the number of design parameters that may be independently varied. Furthermore, the additional manufacturing steps significantly increase the manufacturing cost of arrays that use MUTs.
A further disadvantage associated with the prior art devices shown in FIGS. 1 and 2 is that a relatively large parasitic capacitance may be formed between the one or more MUTs 1 and the underlying substrate 5. Since the MUT 1 is an electro-mechanical device that is generally excited by frequencies in the megahertz range, the formation of parasitic capacitances between the MUTs 1 and the substrate 5 further degrade the performance of the MUTs 1 by producing an additional capacitive load that generally degrades the sensitivity of the MUT.
Accordingly, there is a need in the art for micro-machined ultrasonic transducer structures that are capable of producing significant reductions in acoustic wave propagation in the underlying substrate. Further, there is a need in the art for a micro-machined ultrasonic transducer structures that suppress parasitic capacitive coupling between a MUT and an underlying substrate.
The invention is directed towards improved structures for use with micro-machined ultrasonic transducers (MUTs), and methods for fabricating the improved structures. In one aspect, a MUT is formed on a substrate and an acoustic cavity is formed within the substrate at a location below the MUT. The acoustic cavity is filled with an acoustic attenuation material to absorb acoustic waves propagated into the substrate, and to reduce the effect of parasitic capacitances on the operation of the MUT. In another aspect, the acoustic cavity is formed below a plurality of MUTs that comprise an array. The acoustic cavity and the acoustic attenuation material substantially reduce cross coupling between the MUTs by preventing wave propagation in the substrate. In still another aspect, a plurality of MUTs abut a dielectric layer with the MUTs being substantially encapsulated by the acoustic attenuation material. In yet another aspect, at least one monolithic semiconductor circuit is formed in the substrate that may be operatively coupled to the MUTs, the circuit being positioned in a non-etched portion of the substrate. In still another aspect, the at least one monolithic semiconductor circuit is formed in the substrate and positioned in a thin substrate layer above the acoustic cavity. In yet another aspect, a plurality of MUTs is attached to one side of a layer of semiconductor material, and a dielectric layer is formed on the opposing side. At least one monolithic semiconductor circuit is formed in the semiconductor material that may be operatively coupled to the MUTs.